<p> Improvements in Sub-harmonic mixers and harmonic mixers for use up to 900GHz Hugh J.E. Gibson*, Achim Walber* Byron Alderman† and Oleg Cojocari1</p><p>*RPG Radiometer Physics GmbH / Gibson Microwave Design Meckenheim, Germany Email: [email protected]</p><p>†Rutherford Appleton Millimeter-wave Technology, UK Email : [email protected]</p><p>1ACST GmbH Darmstadt, Germany Email: [email protected]</p><p>Abstract Harmonic mixers for high frequencies have been around for many years but conversion loss has always been relatively high and usually with many dips in frequency response. There are many devices covering the lower waveguide bands, but relatively little above 110GHz. Problems include very poor RF input match, poor temperature stability, extreme sensitivity to LO pump levels and generation of unwanted harmonics. Seeking to overcome these problems, several new designs of harmonic mixer have been developed by RPG Radiometer Physics, as part of Rohde & Schwarz vector network analyser (VNA) waveguide frequency extender series. Flat, high frequency sources, high quality directional couplers and low-loss isolators have also been developed as part of this effort. All components must cover at least a waveguide band, with good performance to the band edges and remain unconditionally stable for maintaining a valid calibration. High performance harmonic mixers are also required for phase-locking high frequency oscillators and spectrum analyzer extenders as well as for simple, compact receivers for airport scanners/FMCW radar. Our latest designs are all balanced mixers, using anti-parallel diodes, either in a cross-bar configuration (like a waveguide doubler) or as an anti-parallel pair. Significant improvements at high frequencies have been achieved by separate collaborations between RPG Radiometer Physics and diode fabrication labs at RAL and ACST to fabricate small, mass produced, extremely low parasitic, anti-parallel diodes which allow reasonable mixer performance up to 900GHz. These anti-parallel diodes are of interest also to sub-harmonic mixer designers (using the 2nd harmonic) as they also achieve outstanding performance at frequencies over 300GHz, where the currently available anti- parallel diodes are starting to show limitations. They can also be used in low-power balanced triplers. We will present the latest designs of our harmonic mixers and show how the problems are overcome. Our solutions include a new modified form of a RF waveguide coupler and the use of photonic band-gap filters to provide low-frequency, wide-band filtering on quartz substrate suitable for 900GHz. Measured test data and simulations will be presented for a range of whole-waveguide band mixers up to 900GHz, which are used in the new range of Rohde & Schwarz’s high frequency VNA extenders.</p>